The typical or conventional method of preparing lead-acid battery plates involves, in general, five steps. First, a battery grid of metallic lead, or more typically, a lead-based alloy, having the desired configuration is made by conventional techniques, such as direct casting or by mechanical working. Second, a leady oxide paste originating with a leady oxide containing, for example, 55-85 wt. % lead oxide and 45-15 wt. % metallic lead is applied to the battery grid, which acts as a support for the wet paste, to prepare the plate. The paste contains what may be considered as the active material precursors for the lead-acid battery. In addition, and as is known, the paste may contain a variety of auxiliary agents, such as, for example, expanders and the like. Third, the freshly pasted plates are briefly exposed to the elevated temperature of a flash drying oven or the like to effect a surface flash drying of the plates to prevent plate-to-plate sticking when the plates are stacked together as typically occurs in subsequent processing.
Fourth, the pasted plates, still internally wet, are cured and allowed to dry in a stacked condition under strictly controlled conditions. The method of plate curing most generally utilized is known as the "hydroset" process. As is fully explained by N. E. Hehner in Chapter 3 ("Curing of Freshly Pasted Plates") of the publication Storage Battery Manufacturing Manual (Independent Battery Manufacturers Association, Inc., Largo, Fla. (1970)), the hydroset process essentially consists of exposing the surface flash dried and stacked plates to temperatures in the range of about 16.degree.-38.degree. C. at relative humidities approaching about 100%. This curing and drying process generally takes about 48-72 hours to complete.
Several changes are generally believed to occur during curing. Oxidation of the residual metallic (free) lead in the paste takes place such that the final amount of free lead in the cured paste is about 3-5 wt. %. The plates dry out so that the final water content in the paste is less than about 1 wt. %. Corrosion processes occuring at the surface of the grid help the paste adhere thereto, and, most importantly, the microstructure of the active material precursor changes to yield a mechanically strong plate.
It is accordingly generally considered that high mechanical strength (i.e., high initial plate strength, high resistance to blistering and spalling during formation, and high resistance to shedding during service life) lead-acid battery plates can only be attained by the oxidation of the free lead in the paste prior to cell or battery formation.
It is further generally believed that the most rapid oxidation of the free lead in the paste prior to cell or battery formation takes place through the use of the aforementioned time-consuming and strictly controlled curing and drying process.
The mechanical strength of lead-acid battery plates is also believed to depend on the formation of basic lead sulfate compounds from lead oxide during the pasting step followed by the dehydration of these basic lead sulfate compounds during the curing and drying of the plates prior to cell or battery formation. Specifically, during the pasting process, only a small amount of the total free lead in the paste is converted into lead oxide while the existing lead oxide of the paste is primarily converted into hydrated tribasic lead sulfate. During the curing and drying of the plates, the hydrated basic lead compounds are converted to tribasic lead sulfate (and/or tetrabasic lead sulfate depending upon curing conditions) with a further conversion of any remaining free lead to lead oxide and elimination of water from the plates.
S. C. Barnes, M. E. D. Humphreys, and R. Taylor in the publication The Curing of Lead-Acid Battery Plates (International Power Sources Symposium, Brighton, England (1968), reprinted by Pergamon Press Ltd.) report upon their investigation of the plate curing process with the attendant reduction of the amount of free lead in the paste and the increase in mechanical strength of the plate and ways of accelerating it by means of closer control of the processing conditions. Their conclusion is that the highest rate of oxidation of the free lead occurs when the paste contains 7-8.5 wt. % water and is cured in an atmosphere maintained at 30.degree. C. and 100% relative humidity. Such a strict control of the processing parameters, designed to ultimately dry out the paste and reduce the free lead content therein to approximately 4 wt. % prior to plate formation, however, is reported to merely reduce the curing and drying time from about 48-72 hours to about 24 hours or more to obtain a plate of equal mechanical strength.
After the plates have been cured and dried, the fifth and last plate preparation step is reached, namely the formation of the battery plates and the conditioning or cycling of the plates. Plates, as is well known, may be formed independently, assembled into cell elements and formed, or assembled into a completed battery and formed. Occasionally, to minimize the accumulation of the potentially hazardous lead dust in the air, the dry plates are wetted with water prior to any handling needed to prepare the plates for formation, e.g., prior to introduction into cell elements and insertion into battery containers.
Formation is usually carried out by passing current through the plates in the presence of a formation electrolyte. Most typically, either a one or two step formation is used, as is known. In the one step formation, the formation electrolyte has a relatively high specific gravity, e.g.--on the order of 1.200 or so. In the latter, the formation electrolyte has a relatively low specific gravity, e.g.--on the order of 1.060 or so. This formation electrolyte is then discarded, and a development electrolyte of higher specific gravity is added. In both cases, the specific gravities of the various electrolytes employed are selected to provide a full charge specific gravity for the electrolyte generally in the range of 1.230 to 1.290, a specific gravity of 1.265 being the typical target. The desired end result is a positive plate containing lead dioxide as the active material and a negative plate containing metallic lead as the active material.